The present invention relates to a method which utilizes at least two energy bands of x-rays to inspect objects subject to x-ray inspection (e.g., baggage) for a specific substance which can be characterized by a specific composite atomic number `Z`.
It is known to inspect objects using single energy x-rays to present an image of the objects with shading representing varying degrees of mass density (opaqueness to x-rays).
It is known to inspect objects using a dual-energy x-ray system and to present an image with color proportional to the ratio of one energy band to the other on a pixel by pixel basis.
Such x-ray inspection systems incorporate an x-ray single or dual energy source, a conveyor, slider, or scanning system to scan the object to be inspected with the x-ray beam, a detector or array of detectors that convert the x-ray flux which penetrates the object to be inspected into pixels of values which represent the x-ray image(s) resulting from the scan, a computing element which may optionally filter, average, enhance, contrast adjust, or otherwise process the image of values output from the detector system, and a display system which presents a black and white or color display of the resulting image of pixel values.
It is known to employ a dual energy x-ray inspection system and to use the value of the ratio of one energy band's attenuation values to the other to discriminate between low-Z material (plastics) and high-Z materials (metals) and to display the respective values as a color difference in the displayed image. These systems display the resulting high-Z to low-Z color-encoded image based on the value of the ratio of the attenuation values at the high and low bands of x-rays for each pixel at a time repeated for every pixel in the image. This method is unable to distinguish composite materials such as metal over plastic and in these cases, an intermediate color is displayed.
The discrimination ability of these systems is insufficient to discriminate adequately between different types of plastics or plastic-like materials, and in the presence of overlying materials, is unable to accurately indicate the presence of specific plastics or metals.